How about a wifi receiver? Lets you stream music from your hard drive to another room. It's not portable though, you need to plug it into the power outlet. I doubt there exists portable versions because of the battery power required for wifi. But then again, you could always use a pda on a home network if you want a portable solution.

I don't want it to be portable (I wouldn't mind, but I don't need it). But a wifi receiver is no good, I don't want to use a computer, I just want a stand-alone MP3 player that reads from external storage.

Something like this, but without the "all-in-one", just the part that says "separate interface inputs for USB memory sticks carrying audio files" plus the interface.